1. Field of Invention
The invention is related to a screw actuator, comprising an actuating member and a screw mechanism having a screw, nut and rolling elements, one of which screw and nut is rotatably supported with respect to the housing and providing a linear movement of the actuating member with respect to the housing in response to a rotational movement of the motor, and a reduction gear means.
2. Description of Related Art
Such screw actuator is known from GB-A-229 1949. Said known screw actuator comprises a screw mechanism which is supported with respect to the housing by means of a bearing capable of accommodating axial and/or radial loads, e.g. an axial thrust bearing for accommodating the axial forces exerted on the brakepads.
This screw mechanism is a so-called roller screw mechanism. Depending on the application condition constraints i.e. space available, and load, one can select a roller screw or a ball screw type actuator. Specific application considerations for a roller screw is that such roller screw mechanism provides a high power density, which means that within specific dimensional constraints, a relatively high load carrying capacity can be provided. Said carrying capacity however is predominantly related to axial loads. With respect to radial loads, the carrying capacity is less favourable compared to a ball screw. A roller screw mechanism is in general more sensitive with respect to radial loads and misalignment.
Another specific component in a roller screw mechanism is the cage which is necessary to space the rollers. In high speed applications this cage mass results in higher starting torques.
The object of the invention is to provide an improved actuator. This object is achieved in that the nut is fixed with respect to the housing, and the screw is rotatably supported with respect to the housing by means of the rolling elements. Said rolling elements may comprise rollers or balls.
In order to obtain about the same load bearing capacity as in a roller screw mechanism, the pitch diameter of the rolling balls, the ball diameter and its contact angle with screw and nut, and the number of turns should be designed such that appropriate dimensions and the required load carrying capacity are provided. However, as the rolling elements of the mechanism can also act as bearing elements for supporting the rotating screw, no separate bearing is necessary to take up the axial load. As a result, the overall dimensions for a ball screw mechanism may remain limited in order to meet application requirements.
The ball screw mechanism is less sensitive with respect to radial loads, which makes it less vulnerable for misalignments. Also, no cage is needed for recirculation of the balls. Instead, recirculation of the balls may be obtained by means of recirculation tube or hole plug between the first and the last ball row or for each ball turn.
The axial moving and rotating screw according to the invention may be driven by the reduction gear means through a coupling means which allows axial displacements. Said coupling means may comprise a drive shaft accommodated within a bore in the screw, the surface of the drive shaft and the bore having axial grooves which engage each other through balls or splines.
The screw may engage the actuating member through a bearing capable to cope with radial and axial load in order to stop the rotating motion in relation to the moving actuating member.
The reduction gear means is preferably contained in a reduction gear module and the screw mechanism is contained in a screw mechanism module. These modules may be in laterally shifted positions.
The actuating member may be executed as a piston, which is slidably held within a cylinder space of the housing. Said piston can be held non-rotatably by means of a groove and pin assembly. The motor drive module can be mounted in-line with the actuator or in angled position.
In other exemplary embodiments of this invention there may be a continuously variable transmission comprising two pulleys, each having two discs enclosing a V-shaped groove, as well as a belt engaging said grooves, the discs of each pulley being movable towards and away from each other so as to continually change the running radius of the belt, wherein the discs of each pulley are displaceable by means of an actuator according to this invention.
For a right angle position of the motor module, the reduction gear means may comprise one or more reduction steps with at least part of a planetary gear system having a stationary outer ring gear wheel with inwardly pointing gear teeth. In particular, the reduction gear means may comprise satellite gear wheels which mesh with the ring gear wheel and which are accommodated on a carrier connected to a rotary shaft engaging the screw mechanism, and the sun gear wheel of the planetary gear system may be accommodated on a drive shaft of the drive module. This system provides an optimal axial compactness of the application.
The sun gear wheel of the reduction gear means is connected to an angled or right angle gear reduction e.g. a bevel gear which mates with a motor driven bevel pinion. Said sun gear wheel and the bevel gear are carried out as a unitary gear wheel which is supported with respect to the nut of the screw mechanism by means of a rolling element bearing. In order to achieve an appropriate reduction, the pitch diameter of the bevel gear is larger than the pitch diameter of the sun gear wheel. The screw, nut, rolling elements and reduction gear components may be obtained by hard turning.
Furthermore, a sensor fixed on a bearing or near the motorshaft may be provided for detecting rotational and/or translational movements of the screw mechanism or other operating parameters. Also, control means may be provided, said control means having an input for a control signal, e.g. from a brake pedal, and being connected to the sensor for controlling the electric motor on the basis of the control signal and the signal from the sensor. The sensor is in particular suitable for obtaining force feedback, wear compensation and/or maintenance indication.
The actuator according to the invention can be applied for different purposes. In particular, the actuator is suitable for use in a brake caliper for an electrically actuatable disc brake, said caliper comprising an actuator as described before, and a claw piece carrying two opposite brake pads, said actuator comprising a screw and a nut one of which is rotatably supported with respect to the housing by means of an angular bearing, and a reduction gear means.
The invention will further be described with reference to the embodiments of FIGS. 1 and 2.